An ultrasonic probe contains an ultrasonic transducer for transmitting and receiving ultrasonic waves. In one type of ultrasonic probe, ultrasonic transducers are connected, with a signal line, to a transmitting circuitry for transmitting a driving signal, and a high-voltage switch circuit is interposed. The switch circuit enables switching among the ultrasonic transducers, and the driving signal is supplied to the selected ultrasonic transducer. The high-voltage switch circuit operates on a high operating voltage applied from a power supply circuit of an ultrasonic-diagnostic-apparatus main body. If, for some reason or other, the driving signal is supplied without application of the operating voltage, the high-voltage switch circuit may be broken down. Therefore, the main body of an ultrasonic diagnostic apparatus to which an ultrasonic probe containing high-voltage switch circuit is connectable typically comprises a voltage monitoring circuit. The voltage monitoring circuit monitors the supply line of the operating voltage supplied from the power-supply circuit of the ultrasonic-diagnostic-apparatus main body to the high-voltage switch circuit. However, even if the signal line leading to the high-voltage switch circuit is not reliably connected to the probe connector, the operating voltage is kept supplied to the voltage monitoring circuit. For this reason, the voltage monitoring circuit provided in the ultrasonic-diagnostic-apparatus main body cannot sense an error.
Japanese Patent No. 4176892 discloses providing a voltage monitoring circuit for an ultrasonic probe. This type of ultrasonic diagnostic apparatus is featured in that a voltage monitoring circuit can sense an error even if the signal line leading to the high-voltage switch circuit is not reliably connected to the probe connector. However, the voltage monitoring circuit has to be provided for each of ultrasonic probes, and the cost of each ultrasonic probe and the cost of the ultrasonic diagnostic apparatus are inevitably high. This problem occurs not only where the operating voltage of a high-voltage switch circuit is monitored but also where the operating voltage of a device of an ultrasonic probe is monitored.